1. Field of the Invention:
The present invention relates to a video signal transmission and/or reproduction system for a plurality of channels and, more particularly, to a transmission system in which a plurality of original video signals of a plurality of channels are processed in a frame unit at intervals of a predetermined cycle in time sequence through the use of time-division multiplexing operation so as to be converted into a single time-division multiplexed video signal; and to a reproduction system in which a video signal corresponding to one of the plurality of the channels is reproduced on the basis of such time-division multiplexed video signal.
2. Description of the Related Art:
A conventional transmission system is known in which a plurality of, for example four (4), original video signals of channels A, B, C and D shown in FIG. 1(a) through FIG. 1(d) respectively are processed through the use of time-division multiplexing operation to prepare a signal time-division multiplexing video signal of FIG. 1(e) which may be recorded on a suitable recording medium from which a desired one or more of the video signals are reproduced. As seen from in FIGS. 1(a) through 1(e), one frame (referred to as channel frame hereinbelow) of each of the original video signals of the channels A, B, C and D is picked up every four frames of each original video signal, and processed in time sequence through the use of time-division multiplexing operation so as to prepare a signal time-division multiplexed video signal of FIG. 1(e). The thus prepared time-division multiplexed video signal is recorded on a suitable recording medium such as video tapes, video discs and the like.
FIG. 2 shows a conventional reproduction apparatus for reproducing a video signal of a desired channel. For example, the channel A is reproduced from the time-division multiplexed video signal of FIG. 1(e) recorded on the recording medium. As shown in FIG. 2, the multiplexed signal from the recording medium enters a video signal from the recording medium enters a video signal demodulation circuit 1 so as to be demodulated therein, and then enters a one-frame memory 2 and a channel detection circuit 3. In this circuit 3, the channel frames corresponding to the channel A contained in the multiplexed signal are picked by a low-level signal during a period of one frame every four frames, the low-level signal being fed to the memory 2 as a write-enabling signal WE of FIG. 1(f). As a result, only the channel frames of channel A stored in the memory 2 under control by a memory write control circuit 4. The frame memory 2 is then controlled by a memory read control circuit 5 so as to repeatedly issue the channel frames to an adder 6 in which synchronizing pulses are applied to the channel frames thereby to prepare the reproduced video single of FIG. 1(g) which is reproduced with the four frame cycle.
Referring to FIGS. 1(a) through 1(g), there has been shown a frame-editing method for preparing the multiplexed video signal in which the original video signals, of the four channels A, B, C and D are multiplexed in terms of the four frame cycle. However, a field-editing method is also known for preparing a time-division multiplexed video signal.
As described above, in the frame-editing method, an editing cycle of the multiplexed video signal is equal to a period of 4 frames (8 fields). When motion appears in a picture, motion is produced even between two interlaced fields belonging to the same frame. As a result, when the picture includes a motion, the frame containing such picture difference in motion is repeated four times in the reproduced picture to cause vibrative motions. Such vibrative motions seriously impair the reproduced picture quality.
On the other hand, in the field-editing method, an editing cycle of the mulitplexed video signal is equal to a period of 4 fields (2 frames) which is a half period of the editing cycle of the frame-editing method. In addition, in the field-editing method, an identical field is repeated so that there is no fear that the above-mentioned vibrative motions of the picture are produced in the reproduced picture. This is an advantage inherent in the field-editing method, but the field editing method is inferior in vertical resolution of the picture to the frame-editing method.